Reject MIME parts with unsupported encoding

What does this MR do and why?

Contributes to #340366

Problem

Golang mime package skips processing unsupported encodings (see https://sourcegraph.com/github.com/golang/go@0fd0639e4c429e147d33bfc42654fcd651f4449f/-/blob/src/mime/mediatype.go?L247).

Because of that workhorse does not incercept the upload and skip sanitization for filename value.

Solution

Manually detect the charset of the filename* and reject the upload if charset is unsupported.

Screenshots or screen recordings

MR acceptance checklist

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